Automated arthroplasty planning

ABSTRACT

Systems and methods are provided to aid in planning at least a portion of a total knee arthroplasty procedure. The system and method automatically aligns the implant components and the bones according to a desired clinical alignment goal with minimum user input. The system and method further allows the user to adjust the position and orientation of the femur, tibia, or implant in a clinical direction regardless of a pre-adjusted position and orientation of the femur, tibia, or implant. The graphical user interface is provided that includes a three-dimensional (3-D) view window, a view options window, a patient information window, an implant family window, a workflow-specific tasks window, and a limb and knee alignment measures window.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 62/302,770 filed Mar. 2, 2016, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention generally relates to the field of computer-aidedsurgical planning, and more specifically to a computerized method tointuitively plan a total knee arthroplasty procedure.

BACKGROUND

Total knee arthroplasty (TKA) is a surgical procedure in which thearticulating surfaces of the knee joint are replaced with prostheticcomponents, or implants. TKA requires the removal of worn or damagedcartilage and bone on the distal femur and proximal tibia. The removedcartilage and bone is then replaced with synthetic implants, typicallyformed of metal or plastic, to create new joint surfaces.

Computer-assisted surgical systems and patient specific instrumentation(PSI) are gaining popularity as a tool to pre-operatively plan andprecisely execute the surgical plan to ensure an accurate final positionand alignment of an implant within a knee joint of a patient that canimprove long term clinical outcomes and increase the survival rate ofthe prosthesis. In general, the computer-assisted surgical systems andPSI systems include two components, an interactive pre-operativeplanning software program and a computer-assisted surgical device or PSIthat utilizes the pre-operative data from the software to assist thesurgeon in precisely executing the procedure.

Conventional interactive pre-operative planning software generates athree-dimensional (3-D) model of the patient's bony anatomy from acomputed tomography (CT) or magnetic resonance imaging (MRI) image dataset of the patient. A set of 3-D computer aided design (CAD) models ofthe manufacturer's implants are pre-loaded in the software that allowsthe user to place the components of a desired implant to the 3-D modelof the bony anatomy to designate the best position and alignment of theimplant on the bone. The pre-operative planning data is used to eitherfabricate the patient specific instrumentation, or it is loaded and readby a surgical device to assist the surgeon intra-operatively inexecuting the plan. Such data is also of value in positioning a surgicalrobot so as to insure spatial access of the robot to the needed surgicalfield while retaining human access to the same.

However, some pre-operative planning software is limited in a fewregards that inhibit the ability of a user to intuitively plan all sixdegrees of freedom of the implants in TKA. First, the user may have toperform a majority of the planning steps manually. For instance, theuser identifies a majority of the anatomical landmarks on the femur andthe tibia to determine various anatomical references (e.g., themechanical axis). Second, as the user manually adjusts the position andorientation of the implant, a series of sequential rotations andtranslations may cause a subsequent or previous degree of freedom tochange un-intuitively. This is inherently due to the additive changes inthe orientation of the coordinate system of the implant as the implantis sequentially rotated or translated. This is non-intuitive because theuser is trying to accomplish certain clinical alignment goals that aremeasured with respect to three well-established orthogonal planes. Thoseorthogonal planes include the coronal plane to accomplish a desiredclinical varus-valgus, the axial plane to accomplish a desired clinicalinternal-external, and the sagittal plane to ensure the implant fitsproperly for a desired varus-valgus and internal-external alignmentgoal. The clinical alignment goals measured from these planes areimportant because they are a standard in the industry and are used bysurgeons to assess clinical outcomes and implant alignmentpostoperatively.

Another limitation of the conventional pre-operative planning softwareis the inability to allow a surgeon or different surgeons to plan fordifferent alignment goals, automatically. Different surgeons havedifferent implant alignment strategies. For example, for varus-valgusalignment, some surgeons prefer to align the implants to restore themechanical axis of the leg, while others prefer to align the implants torestore the native kinematics of the knee. Likewise, forinternal-external alignment, some surgeons prefer to align the implantwith the transepicondylar axis, while others prefer a native kinematicalignment. Conventional planning software may be limited toautomatically aligning the implant to the bone according to a singledefault alignment goal strategy. As a result, a desired outcome may be apriori impossible. Additionally, in situations where bonecharacteristics are abnormal as to a parameter such as density orstructure, the ability to re-select alignment strategy can affordconsiderably better clinical results.

Finally, the conventional planning software does not allow the user tosimply input each of their alignment goals and have the systemautomatically output a transformation between the implant and the bonethat can be readily used by a computer-assisted surgical system. If thesystem was capable of automatically aligning the implant to the bonewithout any manual user adjustments, it may greatly reduce the timespent creating the pre-operative plan, which saves money for the surgeonand health care facility.

Thus, there is a need for a system and method that automatically alignsan implant to a bone with minimal user input. There is also a need for apre-operative planning method that allows a user to make an adjustmentto the placement of an implant with respect to a bone, in a directioncorresponding to a clinical alignment goal or clinical direction,regardless of a pre-adjusted position and orientation of the implant.There is a further need to provide a system and method thatautomatically aligns an implant to a bone with minimal user input.

SUMMARY OF THE INVENTION

A computerized method is provided for planning an arthroplasty procedureaccording to a user's clinical alignment goals. The method includesproviding a graphical user interface (GUI) and virtual models of thefirst bone and the second bone involved in the arthroplasty procedurevia the GUI, locating a set of anatomical landmarks located on thevirtual models of the first bone and the second bone, and automaticallydetermining, by a processor, three orthogonal planes with respect toeach of the virtual models of the first bone and the second bone usingat least a portion of the anatomical landmarks. The method furtherincludes receiving user selections and re-selection of: an implant froma library of implants, the implant having a first implant for the firstbone and a second implant for the second bone, where each implant forthe femur and the tibia have an associated virtual model of the implant,and at least one clinical alignment goal from a set of alignment goals.The model of the first bone and the second bone are automaticallyaligned to the model of the implant to satisfy the at least onealignment goal. The first bones and the second bones are connected andillustratively include the femur-tibia, femur-pelvis, humerus-scapulapairs, respectively.

A surgical planning system is provided for planning an arthroplastyprocedure according to a user's clinical alignment goals. The systemincludes a workstation having a computer, user-peripherals, and amonitor for displaying a graphical user interface (GUI). The computerhas a processor, non-transient storage memory, and other hardware,software, data and utilities to execute the method for planning a kneearthroplasty procedure according to a user's clinical alignment goals.The peripherals allow a user to interact with the GUI and include userinput mechanisms including at least one of a keyboard, mouse, or atouchscreen capability on the monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further detailed with respect to the followingdrawings. These figures are not intended to limit the scope of thepresent invention but rather illustrate certain attribute thereofwherein;

FIG. 1 depicts a high-level overview of a pre-operative planningworkstation and graphical user interface in accordance with embodimentsof the invention;

FIGS. 2A-2C illustrate examples of 3-D models of the bone in differentviews and the locating of anatomical landmarks thereon in accordancewith embodiments of the invention;

FIGS. 3A-3C depict three orthogonal clinically established referenceplanes for planning a TKA in accordance with embodiments of theinvention:

FIG. 4 depicts a femoral planning stage of the GUI in accordance withembodiments of the invention;

FIGS. 5A-5C depict the alignment of a femoral component on a model of afemur in accordance with embodiments of the invention;

FIGS. 6A-6C depict a condylar axis about with an implant can rotate inaccordance with embodiments of the invention;

FIG. 7 depicts a tibial planning stage of the GUI in accordance withembodiments of the invention;

FIG. 8A illustrates a sequential order of a concatenation of femoraltransforms computed by a processor in accordance with embodiments of theinvention; and

FIG. 8B illustrates a sequential order of a concatenation of tibialtransforms computed by a processor in accordance with embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention has utility as a method and system for performingthe same to aid a user in planning at least a portion of an arthroplastyprocedure, such as a total knee arthroplasty. The system and methodautomatically aligns the implant components and the bones according to adesired clinical alignment goal with minimum user input. The system andmethod further allows the user to adjust the position and orientation ofthe bones that in the context of a knee arthroplasty are the femur,tibia, or implant in a clinical direction regardless of a pre-adjustedposition and orientation of the femur, tibia, or implant.

The following description of the preferred embodiments of the inventionin the context of TKA is not intended to limit the invention to thesepreferred embodiments, but rather to enable any person skilled in theart to make and use this invention. Reference is made herein to theplanning of a total knee arthroplasty but it should be appreciated thatembodiments of the present invention may be applied or adapted to theplanning of other orthopedic surgical procedures illustrativelyincluding total hip arthroplasty, hip resurfacing, unicondylar kneearthroplasty, ankle arthroplasty, shoulder arthroplasty, and other jointreplacement procedures.

It is to be understood that in instance where a range of values areprovided that the range is intended to encompass not only the end pointvalues of the range but also intermediate values of the range asexplicitly being included within the range varying by the lastsignificant figure of the range. By way of example, a recited range from1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

With reference to the figures, FIG. 1 illustrates an embodiment of a TKApre-operative planning workstation 100. The workstation 100 includes acomputer 102, user-peripherals 104, and a monitor displaying a graphicaluser interface (GUI) 106. The computer 102 includes a processor 108,non-transient storage memory 110, and other hardware, software, data andutilities to execute the planning process described herein. The userperipherals 104 allow a user to interact with the GUI 106 and mayinclude user input mechanisms such as a keyboard and mouse, or themonitor may have touchscreen capabilities.

A high-level overview of the GUI 106 is shown in FIG. 1 . The GUI 106includes a three-dimensional (3-D) view window 112, a view optionswindow 114, a patient information window 116, an implant family window118, a workflow-specific tasks window 120, and a limb and knee alignmentmeasures window 122. Each GUI window can be summarized as follows. The3-D view window 112 allows the user to view and interact with medicalimaging data, 3-D bone models, and 3-D implant component CAD models. Theview options window 114 provides widgets to allow the user to quicklychange the view of the models of the bone, models of the implantcomponents, and alignment axes, to a desired view. The patientinformation window 116 displays the patient's information such as name,identification number, gender, surgical procedure, and operating side(e.g., left femur, right femur). The implant family window 118 providesdrop-down menus to allow the user to select and re-select a desiredimplant component from a library of implant components. Theworkflow-specific tasks window 120 includes various widgets to provideseveral functions illustratively including: guiding the user throughoutdifferent stages of the planning procedure; allowing the user to selectand re-select desired alignment goals from a set of alignment goals;allowing the user to adjust the implant component(s) and bone models indesired clinical directions; displaying measured values of the alignmentand position of the component(s) on the bone(s); and displaying asummary of the plan. The limb and knee alignment measures 122 displaysthe alignment and position of the implant components on the bone modelssuch as hip-knee-ankle angle, femoral joint line alignment, and tibialjoint line alignment. Overall, the layout of the GUI provides the userwith a convenient roadmap and visual display to successfully plan a TKA.

Prior to planning the procedure, imaging data of the patient's femur andtibia are obtained using an imaging modality such as computed tomography(CT), ultrasound, or magnetic resonance imaging (MRI). The imaging datais transferred to the planning workstation 100 typically in a digitalimaging and communication in medicine (DICOM) format. Subsequently, a3-D model of the bone is generated. In a particular embodiment, thepatient's bones may be manually, semi-manually, or automaticallysegmented by a user to generate the 3-D models of the bone. One or moreof the bone models can be displayed in the 3-D view window 112, wherethe user can quickly change to a proximal, distal, medial, lateral,anterior, and posterior view of bone model using corresponding widgetsin the view options window 114.

An example of three views of the bone models are shown in FIGS. 2A-2C. Alateral view of femoral bone model 124 is shown in FIG. 2A, a distalview of a femoral bone model 124 is shown in FIG. 2B, and a proximalview of a tibial bone model 126 is shown in FIG. 2C. Anatomicallandmarks 128 from a set of anatomical landmarks are located on thefemoral model and tibial model to aid in planning. A portion of theanatomical landmarks may also provide the location for registrationpoints to be collected intraoperatively to register the bone models to acomputer-assisted surgical system as described in U.S. Pat. No.6,033,415. The set of anatomical landmarks for the femur may include:the femoral head center 128 a, most anterior point in intercondylarnotch 128 b, medial epicondyle, lateral epicondyle, anterolateraltrochlear ridge, anteromedial trochlear ridge, most posterior point onmedial condyle, most posterior point on lateral condyle, most distalpoint on medial condyle, most distal point on lateral condyle, and kneecenter. The set of anatomical landmarks for the tibia may include:midpoint between tibial splines, ankle center, center of medial plateau128 c, center of lateral plateau, tibia tubercle (medial ⅓^(rd)),antero-lateral face, and anteromedial face. The processor may identify aportion of the landmarks automatically using an iteration algorithm asdescribed below. The remaining landmarks may be located by the usermanually by pointing and clicking on the location of the landmark on thebone models. In a specific embodiment, as the user or computeridentifies specific landmarks, those landmarks may be used to provide aspecific view (distal, anterior, medial, lateral) of the bone model toeasily identify a specific subsequent landmark. For example, after thethree clinically standard references planes are automaticallydetermined, the user may choose to identify and locate the center of theknee landmark. The user may click on the prompt, where the view of thebone model automatically shows the distal portion of the knee such theuser may quickly locate the center of the knee.

The user may use other landmark locating tools such as a fitting spheretool 130. A user may adjust the diameter and position of the fittingsphere 130 until the diameter and position approximately matches thediameter and position of a portion of the femoral head. When matched,the center of the fitting sphere 130 defines the femoral head centeranatomical landmark 128 a. In a specific inventive embodiment, theprocessor locates all of the landmarks automatically using thestatistical model that was used to automatically generate the 3-D modelsof the bones from the DICOM data. Once the landmarks have beenidentified, they are accepted by the user and stored.

Three orthogonal planes on the femur are determined by the processor,where each plane corresponds to a clinically established standardreference plane for planning any TKA procedure. These planes include acoronal native plane (XZ) 132, a sagittal plane (ZY) 134, and an axialnative plane (XY) 136, as shown in FIGS. 3A-3C, respectively. Thecoronal native plane 132 is defined by the most posterior point on theposterior medial condyle 140, posterior lateral condyle 142, and lessertrochanter 144, such that the plane touches exactly three points butdoes not intersect the bone. An iterative method to automatically findthe coronal native plane 132 includes:

1. Point 1 (lesser trochanter initial guess);

a. Rotate bone model about axis 1 (î+0ĵ+0{circumflex over (k)} in globalLPS (left, posterior, superior) model coordinates);

b. Find most-posterior (+y) point in global LPS coordinate system; c.Find same point in local coordinates on the bone model;

d. Rotate bone model about axis 1 back to original orientation;

2. Point 2 (posterior medial/lateral condyle initial guess);

a. Rotate bone model about axis 2 (0.577î+0.577ĵ+0.577{circumflex over(k)} in global LPS model coordinates);

b. Find most-posterior (+y) point in global LPS coordinate system; c.Find same point in local coordinates on the bone model;

d. Rotate bone model about axis 2 back to original orientation;

3. Point 3 (posterior lateral/medial condyle initial guess);

a. Rotate bone model about axis 3 (0.577î−0.577ĵ−0.577{circumflex over(k)} in global LPS model coordinates);

b. Find most-posterior (+y) point in global LPS coordinate system;

c. Find same point in local coordinates on the bone model;

d. Rotate bone model about axis 3 back to original orientation;

4. Iteratively update points;

a. Re-orient bone model such that all three points are parallel to theXZ plane;

b. Find most-posterior (+y) point in global LPS coordinate system;

c. Find same point in local coordinates on the bone model (Point 4);

d. Whichever point (point 1, 2, or 3) is closest to Point 4, delete thatpoint and replace it with Point 4;

e. Repeat a-d until points do not change;

The axial native plane 136 is defined as perpendicular to the coronalnative plane on the femur, and coincident with the most distal point onthe distal medial condyle 146 and the most distal point on the distallateral condyle 148. A method to automatically find the axial nativeplane 136 includes:1. Point 1 (distal medial/lateral condyle initial guess);a. Rotate bone model about axis 1 (0î+1ĵ+0{circumflex over (k)} inglobal LPS (left, posterior, superior) model coordinates);b. Find most-distal (−z) point in global LPS coordinate system;c. Find same point in local coordinates on the bone model;d. Rotate bone model about axis 1 back to original orientation;2. Point 2 (posterior medial/lateral condyle initial guess);a. Rotate bone model about axis 2 (0î−1ĵ+0{circumflex over (k)} inglobal LPS model coordinates);b. Find most-distal (−z) point in global LPS coordinate system;c. Find same point in local coordinates on the bone model;d. Rotate bone model about axis 2 back to original orientation;3. Iteratively update points;a. Re-orient bone such that the coronal kinematic plane is coincidentwith the XZ plane;b. Re-orient bone about the Y-axis until both points are parallel withthe XY plane;c. Find most-distal (−z) point in global coordinate system;d. Find same point in local coordinate on the bone model (Point 3);e. Whichever point (point 1 or 2) is closest to Point 3, delete thatpoint and replace it with Point 3;f. Re-orient bone such that the bone is in the original coordinatesystem;g. Repeat a-f until points do not change.The sagittal plane 134 is defined as perpendicular to both the coronalnative plane 132 and the axial native plane 136, and coincident with amedial-lateral center point of the bone. The medial-lateral center pointmay be determined by computing the midpoint between the medialepicondyle landmark 150 and the lateral epicondyle landmark 152. Thesethree orthogonal planes are clinically established standard referenceplanes used for planning any TKA. It should be appreciated that the LPScoordinate system used above is not an essential reference coordinatesystem to determine the planes (132, 134, 136), where other referencecoordinate systems may be used. In addition, the amount to rotate thebone model about the axes 1, 2, and 3 to locate the points that definethe planes may be tuned to ensure convergence for different patientpositions during scanning.

An initial femur transform is determined using the intersection of thethree orthogonal planes to establish a position and orientation of thex, y, and z axes relative to the bone. The initial femur transformprovides the basis for aligning and positioning the implant componentsto the bone models in the clinically established standard referenceframe.

With reference to FIG. 4 , a workflow-specific tasks window 120 is shownfor the femoral planning stage of the planning procedure. Briefly, asshown on the left side of the tasks window 120 are tabs 154-158 whichallow the user to toggle between the different planning stages of theprocedure (i.e., landmarks stage 154, femur planning stage 155, tibiaplanning stage 156, summary stage 157, and a surgery planning stage158). FIG. 4 depicts the tasks window in the femoral planning stage 155.The femoral planning stage includes an implant drop-down menu 160, acoronal alignment goal drop-down menu 162, an axial alignment goaldrop-down menu 164, a distal bone resections sub-window 166, a posteriorbone resection sub-window 168, a flexion sub-window 170, and amedial-lateral sub-window 172. A user may select and re-select a desiredimplant from a library of implants using the implant drop-down menu 160.A user may select and re-select a varus-valgus alignment goal from a setof varus-valgus alignment goals using the varus-valgus alignmentdrop-down menu 162. Likewise, a user may select and re-select an axialrotational alignment goal from a set of axial rotation alignment goalsusing the axial alignment drop-down menu 164. The set of femoral coronalalignment goals may include a native alignment and a neutral mechanicalaxis. The set of femoral axial alignment goals may include: parallel tothe transepicondylar axis; an angle offset from an anatomical axis(e.g., 1°-10° from the posterior condylar axis); and a native alignment.

Each of the sub-windows 166, 168, 170 and 172, allow the user to adjustthe implant or bone in four clinical directions. The four clinicaldirections include a proximal-distal translational direction (sub-window166), an anterior-posterior translational direction (sub-window 168), amedial-lateral translational direction (sub-window 172), and aflexion-extension rotational direction (sub-window 170). The directionsare referred to as clinical because the user can adjust each directionindividually, and an adjustment in the clinical direction corresponds toa direction with reference to the clinically established referenceframe, regardless of a pre-adjusted position and orientation of theimplant or bone as described below. The user can adjust the clinicaldirection using the corresponding “+” button 174 or “−” button 176.Reset buttons 178 allow the user to reset any adjustments to a defaultvalue. A measured amount of distal resection on the medial and lateraldistal condyles 180 is displayed as the user adjusts any clinicaldirections and/or selects/reselects an implant or alignment goal.Similarly, a measured amount of posterior bone resection on the medialand lateral posterior condyles 182 is displayed as the user adjusts anyclinical directions and/or selects/reselects an implant or alignmentgoal. In a particular inventive embodiment, an additional sub-windowallows the user to adjust for an estimated or measured cartilagethickness, cartilage wear, or bone wear to translate the implantaccordingly.

A default femoral coronal alignment goal, and femoral axial alignmentgoal are pre-set when the user enters to the femur planning stage. Auser then selects a femoral implant component, and the femoral bonemodel is automatically aligned to the selected implant according to thedefault alignment goal. The default alignment goals may be a nativefemoral coronal alignment and a native femoral axial alignment. Theprocessor automatically aligns the bone to the implant using the initialfemur transform, the native alignment goals, and a portion of thegeometry of the implant. An example of a femoral implant component 184is shown in FIG. 5A. The femoral implant 184 includes a distal planarsurface 186 to interact with a distal bone cut and a posterior planarsurface 188 to interact with the posterior bone cut. The planar surfaces(186, 188) are also referred to herein as implant cut planes. A femoralimplant distal articular plane 190 is defined as an offset plane fromthe distal planar surface 186 by a maximum thickness of the distalportion of the implant. Likewise, a femoral implant posterior articularplane 192 is defined as an offset plane from the posterior planarsurface 192 by a maximum thickness of the posterior portion of theimplant. The articular planes (190, 192) are also referred to herein asoffset implant cut planes. The processor then automatically aligns theimplant to a native alignment goal by aligning the axial native plane136 to the implant distal articular plane 190, and the coronal nativeplane 132 to the implant posterior articular plane 192. FIGS. 5B and 5Cdisplay the result of the femur model 124 and the implant 184 aligned innative alignment.

When a user selects a mechanical axis coronal alignment and a non-nativeaxial alignment for the femur, projection angles rather than directioncosines are used to build a rotational adjustment transform to align theimplant and the bone. By using projection angles, individual degrees offreedom can be changed/adjusted without substantially affecting theother degrees of freedom and subsequent changes/adjustments do notsubstantially affect previous adjustments/changes. In one inventiveembodiment, substantially affecting the other degrees of freedom refersto 1 mm or 1 degree. In another inventive embodiment, the substantiallyaffecting refers to 0.5 mm and 0.5 degrees. While in other inventiveembodiments, the substantially affecting refers to 0.1 mm and 0.1degrees.

For example, when a user selects a mechanical axis alignment, themechanical axis (defined as an axis connecting the femoral head centerand the center of the knee) is projected onto the coronal native plane132. The angle between the z-axis and the projected mechanical axis isdetermined and used to build a portion of the rotational adjustmenttransform. Simultaneously, when a user selects, for example, thetransepicondylar axis (defined as an axis connecting the medial andlateral epicondyles) is projected onto the axial native plane 136. Theangle between the x-axis and projected transepicondylar axis isdetermined to build the second portion of the rotation adjustmenttransform. The user can make any adjustments in the other clinicaldirections as desired.

In a particular inventive embodiment, with reference to FIGS. 6A-6C, theprocessor determines a condylar axis 194 where any adjustment in theflexion-extension rotation direction occurs about the condylar axis 194.The condylar axis 194 may be determined by fitting one circle to aportion of the medial condyle 198, and fitting a second circle to aportion of the lateral condyle. Each circle having a center 200. An axisconnecting the two centers of the circles define the condylar axis 194.In another inventive embodiment, the condylar axis 194 is determined asa center axis of a cylinder 202 traversing through the medial condyle198 and the lateral condyle 204, where the diameter of the cylinder isbest-fitted to a portion of the condyles. In another inventiveembodiment, a sphere is fitted to the medial 198 and lateral condyles204, wherein an axis connecting the center of the two spheres definesthe condylar axis 194. In a specific inventive embodiment, the condylaraxis 194 is determined by the following:

1. Map articular surface;

a. Re-orient bone model such that the coronal native plane is coincidentwith the XZ plane, the axial native plane is coincident with the XYplane, and the sagittal native plane is coincident with the YZ plane;

b. Rotate bone about global X-axis (flexing the knee) about flexionincrement;

c. Locate the most-distal (−z) point in global coordinate system thathas a positive x-coordinate (condyle 1 articular surface point). Locatesame point in local bone coordinates.

d. Locate the most-distal (−z) point in global coordinate system thathas a negative x-coordinate (condyle 2 articular surface point). Locatesame point in local point coordinates:

e. Increment flexion and repeat c-d;

f. Repeat e through entire range of flexion;

2. Fit cylinder to mapped articular surface:

3. Center axis of the cylinder is the condylar axis 194.

As shown in FIG. 6C, as the implant or bone is rotated inflexion-extension about the condylar axis, the posterior resection 206and the distal resection 208 does not substantially change because theimplant is essentially rotating about the semi-circular condyles.Therefore, the user can set the anterior portion 210 of the implant suchthat there is no notching without affecting these resections.

After the femur has been planned, the user may plan the tibia, althoughthe user may go back and forth between the femur and tibia planningstages. The tibia component is automatically aligned to the femoralcomponent by matching the articular surface of the tibia to thearticular surface of the femoral component. As shown in FIG. 7 , theuser can select and re-select a tibial baseplate component usingbaseplate drop-down menu 212, and the user can select and re-select atibial liner using tibial liner drop-down menu 214. The user can thenadjust the posterior slop 216, coronal alignment 218, proximal boneresections 220, axial rotation 222, and translational position 224. Ameasured value of the tibial slope 226 is displayed and a measure valueof the tibial proximal bone resections 228 is also displayed. Theposterior slope is adjusted in flexion-extension about the condylar axis194 defined on the femoral condyles as described above. The axialrotation occurs about a projection of the mechanical axis of the tibiaonto the sagittal native plane 134.

FIGS. 8A and 8B depict a sequential order of a concatenation oftransforms that allow the user to make an adjustment in any clinicaldirection and/or select/re-select an alignment goal, such that theimplant is re-aligned in the desired clinical direction or there-selected alignment goal, regardless of a pre-adjusted position andorientation of the implant. This specific order of transforms isdesigned such that the adjustments are the inputs and the outputs. Whena user makes an adjustment in a clinical direction, the model istransformed in the desired clinical direction. After the model has beentransformed, the position and orientation of the implant is measuredwith respect to the bone, which directly correlates to adjustment made.This order is computationally fast and provides the user with anintuitive tool for aligning an implant with respect to the boneaccording to a clinical standard reference frame.

FIG. 8A depicts the sequential order of the concatenation of transformscomputed by the processor for the femoral planning stage to intuitivelyalign and position the femoral implant to the bone. The anatomicallandmarks are determined (Block 230) and an initial femoral basetransform is determined (Block 232). The second transformation isbetween the femoral implant component (Block 234) and the femur model(Block 236). The third transformation is an adjustment transform forfemoral coronal and axial rotation (Block 242). The fourth transform isa translation transform for any adjustment in the anterior-posterior,proximal-distal, medial-lateral, and an optional account for cartilagethickness (Block 252). The final transform is an adjustment in theflexion-extension rotation direction about the condylar axis 194 (Block256). Note, each of these transforms have default values, which allowthe user to adjust any of the alignment goals or clinical directions inany sequence, however the transforms are computed in this sequentialorder to allow for any adjustments to occur in the desired directionregardless of the position and orientation of the implant or bone.

FIG. 8B depicts the sequence of the concatenation of transforms computedby the processor for the tibial planning stage to intuitively align andposition the tibial implant to the bone. A first initial tibia transform(Block 258) from the anatomical landmarks. A second transform betweenthe tibial component and the femoral component (Block 262). A thirdrotation transform in flexion-extension rotation about the condylar axis194 (Block 266). A fourth transform in a varus-valgus alignment (Block270). A fifth transform in internal-external rotation where the rotationoccurs about the projection of the mechanical axis of the tibia to thesagittal native plane (Block 274). A final translation transform for anyadjustments in a translational direction (Block 282).

The sequential order of the concatenation of transform allows the userto make adjustments to the implant or bone in an intuitive manner. Oncethe femur and tibial planning stages are complete, the user can reviewthe hip-knee-ankle angles, femoral joint line alignment, tibial jointline alignment, femoral distal resections, femoral posterior resections,proximal tibial resections, and posterior slope, as well as patientinformation and surgical procedure information in the summary stage ofthe procedure. In the surgical planning stage, the user can determinewhich bone should be operated on first and define any parameters for acomputer-assisted surgical system. The final plan is accepted by thesurgeon and is written to a data transfer file (e.g., compact disc (CD),portable universal serial bus (USB)) for use with the computer-assistedsurgical system. The final plan includes the final femur-to-implanttransform, and the final tibia-to-implant transform to register andexecute the TKA according to the plan.

In a particular inventive embodiment, if a user does not deviate from aparticular planning strategy, the user may save their set of alignmentgoals in the planning workstation that can be applied to all surgicalcases. Due to the minimal user input required by the surgeon, the savedpreferences can improve pre-operative planning times.

In a specific inventive embodiment, if a user desires a nativealignment, the pre-operative planning may be performed nearlyautomatically. The coronal native plane, axial native plane and sagittalnative plane may be determined as described above. The bone wear may beaccounted for by the following:

a. Varus Malalignment

-   i. Without changing the tibia or the kinematic planes, rotate the    femur in the coronal plane about an axis perpendicular to the    coronal native plane and coincident with the most distal point on    the lateral condyle. Rotation amount should be the arctangent of the    bone wear on the lateral side divided by the distance between the    two most distal points on the femur, or

$\theta = {{atan}\left( \frac{t_{{wear},{bone}}}{d_{{medial}\mspace{14mu}{to}\mspace{14mu}{lateral}\mspace{14mu}{condyle}}} \right)}$b. Valgus Malalignment

-   i. Without changing the tibia or the kinematic planes, rotate the    femur in the coronal plane about an axis perpendicular to the    coronal native plane and coincident with the most distal point on    the medial condyle. Rotation amount should be the arctangent of the    bone wear on the medial side divided by the distance between the two    most distal points on the femur, or

$\theta = {{atan}\left( \frac{t_{{wear},{bone}}}{d_{{medial}\mspace{14mu}{to}\mspace{14mu}{lateral}\mspace{14mu}{condyle}}} \right)}$

The cartilage wear may be accounted for by the following:

a. Varus Malalignment

-   i. Without changing the tibia or the kinematic planes, rotate the    tibia in the coronal plane about an axis perpendicular to the    coronal native plane and coincident with the most distal point on    the lateral condyle. Rotation amount should be the arctangent of the    total cartilage wear on the lateral side divided by the distance    between the two most distal points on the femur, or

$\theta = {{atan}\left( \frac{t_{{wear},{cartilage}}}{d_{{medial}\mspace{14mu}{to}\mspace{14mu}{lateral}\mspace{14mu}{condyle}}} \right)}$b. Valgus Malalignment

-   i. Without changing the tibia or the kinematic planes, rotate the    femur in the coronal plane about an axis perpendicular to the    coronal native plane and coincident with the most distal point on    the medial condyle. Rotation amount should be the arctangent of the    total cartilage on the medial side divided by the distance between    the two most distal points on the femur, or

$\theta = {{atan}\left( \frac{t_{{wear},{cartilage}}}{d_{{medial}\mspace{14mu}{to}\mspace{14mu}{lateral}\mspace{14mu}{condyle}}} \right)}$

The femoral implant size may be determined using the medial-lateralwidth and femoral anterior-posterior size of the femur. The femoralimplant is placed on the bone model such that the articular surface ofthe femoral component contacts the coronal native plane and axial nativeplane. The two posterior condyles of the component contact the coronalnative plane in exactly two points such that the component does notintersect the plane. The two distal condyles of the component contactthe axial native plane in exactly two places such that the componentdoes not intersect the plane. The femoral implant is automaticallyrotated in flexion-extension about the condylar axis 194, maintainingthe requirements of the placement, until the most proximal part of theanterior surface of the femoral implant is on the anterior surface ofthe bone (no notching). The tibial component comes in linked to thefemoral component at full extension and the flexion of the tibia iscorrected about the femoral component flexion axis coincident with thecondylar axis 194.

OTHER EMBODIMENTS

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of thedescribed embodiments in any way. Rather, the foregoing detaileddescription will provide those skilled in the art with a convenientroadmap for implementing the exemplary embodiment or exemplaryembodiments. It should be understood that various changes may be made inthe function and arrangement of elements without departing from thescope as set forth in the appended claims and the legal equivalentsthereof.

The invention claimed is:
 1. A computerized method for automaticallyplacing a model of an implant relative to a model of a bone, the methodcomprising: automatically determining three orthogonal planes withrespect to a virtual model of a first bone using a plurality ofanatomical landmarks located on the virtual model of the first bone,wherein the three orthogonal planes comprises a coronal plane, an axialplane, and a sagittal plane of the virtual model of the first bone;receiving selections of: a first implant, and an alignment goal from aset of alignment goals; and automatically placing a virtual model of thefirst implant, corresponding to the selected first implant, on thevirtual model of the first bone to satisfy the selected alignment goalbased on a computation utilizing the selected alignment goal, the threeorthogonal planes, and geometry data of the virtual model of the firstimplant.
 2. The computerized method of claim 1 wherein the computationcomprises computing a concatenation of transforms to align the virtualmodel of the first implant with respect to the virtual model of thefirst bone.
 3. The computerized method of claim 2 wherein a sequentialorder of the concatenation of transforms includes an initial first bonetransform, a first bone-to-implant transform, an alignment goaltransform, a translation first bone transform, and a flexion-extensionfirst bone rotation transform.
 4. The computerized method of claim 2wherein the portion of the geometry of the virtual model of the firstimplant is a plane offset by a thickness of the implant.
 5. The methodof claim 1 wherein the set of alignment goals a comprises varus-valgusalignment goals and axial rotational alignment goals, wherein thevarus-valgus alignment goals comprise a neutral mechanical axis of thefirst bone and a native alignment of the first bone, and the axialrotation alignment goals comprise a parallelity to a transepicondylaraxis of condyles of the first bone an angle offset from an anatomicalaxis of the first bone, and a native alignment of the first bone.
 6. Thecomputerized method of claim 1 further comprising receiving useradjustments of a desired change in at least one of four clinicaldirections, wherein each clinical direction is adjusted independently,and wherein the virtual model of the first bone with respect to thevirtual model of the first implant is adjusted automatically using acalculation to satisfy the desired change.
 7. The computerized method ofclaim 6 further comprising automatically re-placing the virtual model ofthe first bone with respect to the virtual model of the first implant inresponse to at least one of: an adjustment made in a desired clinicaldirection; or a re-selection of an alignment goal; and wherein at leastone of the adjustments and the re-selection is inputted into theconcatenation of transforms; and wherein the concatenation of transformsare computed in a sequential order to re-align the virtual model of thefirst implant with respect to the virtual model of first bone in thedesired clinical direction or the re-selected alignment goal withoutaffecting a pre-adjusted position and orientation of the virtual modelof the first implant with respect to the virtual model of the firstbone.
 8. The computerized method of claim 6 further comprising receivinga user reset of an adjustment made by the user in one or more of theclinical directions to a default value.
 9. The computerized method ofclaim 6 wherein the computer automatically determines a position andorientation of the virtual model of the first implant in six degrees offreedom with respect to the virtual model of the first bone.
 10. Thecomputerized method of claim 6 wherein the four clinical directionsinclude a medial-lateral translation direction, a proximal-distaltranslation direction, an anterior-posterior translation direction, anda flexion-extension rotation direction.
 11. The computerized method ofclaim 1 further comprising determining a condylar axis with respect totwo condyles of the virtual model of the first bone, wherein thecondylar axis is determined by at least one of: an axis connecting thecenter of two circles, where each circle is fitted about a portion ofeach condyle; or a transepicondylar axis of the condyles.
 12. Thecomputerized method of claim 1 wherein the determining of the threeorthogonal planes comprises: iteratively locating three most posteriorpoints on the virtual model of the first bone to define a coronal plane;iteratively locating two most distal points on the virtual model of thefirst bone and computing an axial plane defined as a plane normal to thecoronal plane and coincident with the two most distal points; andlocating a medial-lateral center point on the virtual model of the firstbone, and computing a sagittal plane defined as a plane normal to thecoronal plane, a plane normal to the axial plane, and coincident withthe medial-lateral center point.
 13. The computerized method of claim 1wherein a virtual model of a second implant is automatically placed withrespect to the virtual model of the first implant.
 14. The computerizedmethod claim 1 wherein the virtual model of the first bone is a virtualmodel of at least a portion of a femur bone.
 15. A surgical planningsystem for performing the computerized method of claim 1 the systemcomprising: a computer comprising a processor, non-transient storagememory, and software to execute the method of claim
 1. 16. The system ofclaim 15 further comprising a graphical user interface (GUI) comprising:a three-dimensional (3-D) view window, a view options window, a patientinformation window, an implant family window, a workflow-specific taskswindow, and a limb and knee alignment measures window.
 17. Thecomputerized method of claim 1, further comprising: providing agraphical user interface (GUI); and locating the set of anatomicallandmarks located on the virtual model of the first bone.
 18. Thecomputerized method of claim 1 wherein the virtual model of the firstimplant is selected from a library of implants.
 19. The computerizedmethod of claim 1 wherein the plurality of anatomical landmarks: for thefirst bone being a femur bone include at least one the femoral headcenter, most anterior point in intercondylar notch, medial epicondyle,lateral epicondyle, anterolateral trochlear ridge, anteromedialtrochlear ridge, most posterior point on medial condyle, most posteriorpoint on lateral condyle, most distal point on medial condyle, mostdistal point on lateral condyle, or knee center; and for the first bonebeing a tibia bone include at least one of midpoint between tibialsplines, ankle center, center of medial plateau, center of lateralplateau, tibia tubercle, antero-lateral face, or antero-medial face.